A general rule of thumb is that you irrigate plains and you mine grasslands.
Early in the game...
1) when you are a despot, it does no good to irrigate a grass because of the penalty for being a despot.
2) you want growth over production so you need tiles that produce at least 2 food per turn (RoT)
Middle game...
Generally following the rule of thumb but you'll also have cities that can work tiles to an optimum number of sheilds per turn to produce veteran units. In other words, production takes priority over growth but you still would like the city to continue growing.
Late in the game...
With the componding affect of rails on food/sheild production and a limit on your city size,12 w/o a hospital, you have the two cases of cities that are 1) productive and 2) corrupt.
For the productive cities you'll maximize the sheild production, for the tiles that you can work, with the ideal situation of having zero excess food.
For the corrupt cities you'll maximize food production so you can hire specialists.
I'm sure there are whole articles in the War Academy that hit on this in detail but this is just my $0.02 worth. Enjoy
